The present invention relates to surface acoustic wave devices and communication devices.
In conventional spread spectrum communication devices using the direct spreading method, a Barker code was used as a pseudo noise code as described in 1985 ULTRASONICS SYMPOSIUM proceedings, pp. 145-148, for example. It is known that this code does not depend on the arrangement of an information code sequence and this code has an auto-correlation side lobe of 1.
The Barker code has been found with the code length of 13 or less. The Barker code has not been found with a code length exceeding it. In the case where a processing gain of a code length of at least 14 was required, therefore, a code other than the Barker code, such as the longest code sequence was used. In these code sequences, however, a large side lobe rise is caused when the sign of the information code is inverted. In general, therefore, the error rate is increased.
An object of the present invention is to solve the above described problem and provide a novel structure of a spread spectrum communication device which uses a code sequence having a code length of at least 14, which does not depend on the arrangement of the information code, and which suppresses the side lobe rise of correlation signals.
The above described object can be achieved by employing a code sequence used by the present invention, i.e., a code shown in TABLES 1 through 9 as the pseudo noise code for spreading the power density spectrum of an input signal.
It has been confirmed by calculation conducted by the present inventors that the code shown in TABLES 1 through 9 has an auto-correlation coefficient side lobe of 3 or less. If this code is used, therefore, there is obtained a novel spread spectrum communication device and communication system which has a processing gain having an auto-correlation coefficient side lobe of at least 14, which does not depend on the arrangement of the information code, which suppresses the side lobe rise of the correlation signal, and which makes the error rate small. In addition, a surface acoustic wave device utilizing this characteristic is obtained.
The present invention relates to a novel code having a processing gain with a code length of at least 14 and an auto-correlation side lobe of 3 or less. The code length is determined in some cases by using harmonics of the crystal (oscillation frequency in the case where a frequency multiplier is used). In other cases, the code length is generated independently of the clock frequency of the baseband digital circuit.
It is now assumed that the code length is determined by using harmonics of the crystal. If harmonics are generated by distorting the oscillation waveform of the clock frequency in order to make harmonic components large, for example, only odd-number components included in harmonic components are typically generated. If its odd-number components are used as the clock of pseudo noise code generator, therefore, a value obtained by dividing the clock of the pseudo noise code generator by the clock frequency of the baseband digital circuit, i.e., the code length is obtained. To be concrete, an odd-numbered code length such as 15, 17, 19, . . . is obtained.
In the case where the obtained odd-numbered code length is applied to a spread spectrum communication device, two code lengths are combined and used in some cases. To be concrete, it is a result of multiplication of odd-numbered code lengths. A value of at least 14 such as 15, 21, 25, 27, . . . is used as the code length.
In the case where the code length is generated independently of the clock frequency of the baseband digital circuit, it doesn""t matter at all whether the code length is even-numbered or odd-numbered and consequently values of at least 14, i.e., values 14, 15, 16, 17, . . . are obtained.
In either case, a pseudo noise code formed by a large number of combinations in respective code lengths is present. Out of them, the present inventors found a novel code which is 3 or less in auto-correlation side lobe and which is effective as the pseudo noise code used for spreading the power density spectrum of an input signal. The present inventors also found a novel pseudo noise generator capable of executing spreading (or de-spreading) of the power density spectrum of an input signal by using those codes.
The following TABLES 1 through 9 show the pseudo noise code concerning the present invention.